Exhaust gasses are a by-product of many industrial processes and may contain a variety of substances such as lead, cadmium, zinc, iron, and/or dioxins. Many of these substances are considered harmful to the environment and need to be cleaned before an exhaust gas may be safely vented to the atmosphere. These substances, though harmful to the environment, may be reused as raw materials as long as their concentrations in the collected solid wastes are sufficiently high. When removed from the exhaust gas, and, in the same time, separated from each other, these substances may find economic use in the originating process or at an outside facility. Therefore, the efficient treatment of exhaust gases and separation of useful substances in the process has important environmental and financial implications.
For example, the process for steel-making using a basic oxygen furnace will create an off-gas containing a number of components, including iron and zinc. After scrap steel is charged into a basic oxygen furnace, molten iron from a blast furnace is poured into the furnace, and high quality oxygen is injected into the furnace, typically using a water-cooled oxygen lance. The introduction of oxygen at high speeds causes oxidation of carbon, other impurities, and some iron in the mixture, resulting in heat production and rapid mixing. In the process of blowing oxygen, some additive materials, like flux and alloy, are added into the furnace. The scrap steel often contains zinc, which can easily evaporate in the steelmaking process. Because of the strong turbulence and the high temperatures associated with making steel, 10-30 kg dust per ton liquid steel can be generated from ejection of liquid slag and molten iron, vaporization of evaporable components like zinc and lead, and entrainment of additive materials. The dust is carried out with the exhaust gas.
Previous methods of treating such gases have not considered to make the exhaust gas solid wastes recyclable in the gas cleaning process. Often the exhaust gas is treated by wet scrubbers, or initially treated by gravity dust catchers and then electrostatic precipitators. This generates a mixture of dust or a sludge which often contains zinc. The zinc level in the dust or sludge is often too high to reuse in an iron or steel-making process but too low to economically treat at a zinc recovery facility. Consequently, the dust or sludge is often disposed to landfills, or occasionally recycled using a subsequent process at great expense. These subsequent processes typically mix the dust or sludge with carbon or a carbon containing substance to form a mixture which is turned into pellets or briquettes. This mixture is subsequently processed using a number of different steps, such as heating, melting, volatilization, and reoxidization, to separate out various substances.
These prior methods, however, suffer from a number of drawbacks. Disposal of the non-recyclable solid wastes is increasingly costly and may be not allowable. Combining the solid wastes with carbon or a carbon material adds expense and additional processing steps, both to form the mixture and then to separate the initial substances back out. The heating, melting, and/or volatilization of these mixtures require a large amount of heat input, resulting in additional expense, wasted resources, and further pollution. Additionally, these types of processes, as well as other methods, may not effectively remove all of the harmful substances from the exhaust gas. Prior methods are also ineffective at properly isolating and separating useful substances.